Cell surface receptors for viruses and polypeptide hormones were studied in cells of murine and human origin. The human membrane receptors for Encephalomyocarditis virus were found to be quantitatively and qualitatively different from those on murine cell membranes, although cells of both species are susceptible. Another class of membrane receptors, those for the polypeptide hormone insulin, was studied following infection with Herpes Simplex and other viruses. The number of insulin recptors decreased following herpes simplex and vesicular stomatitis viruses but not after infection with measles and Sinbis viruses.